It is often necessary to lift or lower loads offshore. For example a lifting apparatus on a floating vessel may be employed to lift a load rHsting on the seabed with a rope extending from the lifting apparatus to the load. In a typical arrangement the rope is wound on a winch mounted on the vessel and passes over one or more sheaves rotatably mounted on the lifting apparatus. The rope extends downwardly from the final sheave to the load to be lifted. Especially when lifting heavy loads it is important to avoid sudden changes in the tension of the lifting rope and a lifting operation typically proceeds slowly. A particular difficulty that may arise, however, as a result of heave of the floating vessel, is that there is relative vertical movement between the floating vessel and the load to be lifted; without any compensation for such relative movement, very substantial forces can arise in the rope.
If the load has not yet been lifted from a rest position but the rope is already under tension, and the vessel is caused by the sea to rise, then the rope tension increases substantially as the rope seeks to impart a substantial upward acceleration to the load which has high inertia. If the load has already been lifted from a rest position, and the vessel is caused by the sea to rise, then the rope resists that and again tension in the rope increases considerably. If the load has already been lifted from a rest position, and the vessel is caused by the sea to descend, then the rope tension decreases substantially as the load will not descend as quickly as the vessel due to drag (the hydrodynamic forces on the load). In this situation there is a risk of rope slack and subsequent shock loading of the rope. Consequently it is common to provide some kind of heave compensation system when lifting a load.
In a known heave compensation system, where a rope passes from the lifting apparatus to the load, it is known to adjust the effective length of the rope to compensate for heave. For example the rope may extend from the load over a rotatable sheave and then along a serpentine path defined by further sheaves, one of which may be drivingly movable to make the path more or less serpentine. Taking as an example the case where the lifting apparatus sinks, then, to avoid a sudden reduction in tension in the rope, the movable sheave is driven to make the rope path more serpentine so that the serpentine portion of the rope path is lengthened; therefore rope is caused to pass over the rotatable sheave in the direction from the load to the serpentine path. Similarly, if the lifting apparatus rises, to avoid a sudden increase in rope tension, the movable sheave is driven to make the serpentine portion of the rope path shorter, and rope is caused to pass over the rotatable sheave in the direction from the serpentine path to the load. In that way, the variation in the tension in the cable can be reduced or eliminated, but the cable is exposed to reciprocating movement over a sheave with a period equal to the period of the heave of the vessel.
Such reciprocating movement over a sheave of a cable that is under substantial tension tends to cause fatigue in the cable and, to accommodate that, a higher specification of rope is required.
The present invention seeks to mitigate the above-mentioned problem arising from fatigue in a lifting rope.